This invention relates to electrolytic etching and milling of metal alloys, particularly nickel superalloys.
Nickel superalloys, such as are used for high temperature service in gas turbines, are quite difficult to machine by conventional processes. Electrolytic etching is a particularly attractive method for removing small quantities of such metals, especially when small depressions or grooves are sought on the surface of a workpiece. Generally, electrolytic etching comprises the selective removal of portions of a workpiece by the combined action of an electric current and a corrodent. The conventional method of electrolytic etching involves immersing the workpiece in an electrolyte with an electrode and applying an electric potential so that the workpiece is anodic. Metals vary, of course, in their susceptibility to electrolytic etching. Many advanced nickel superalloys are by metallurgical design resistant to corrosive elements, including the acids normally used for electrolytic etching. They are complex, multi-phase materials and it is often found that the different phases vary in their rates of removal. Conventional electrolytic etching of the more highly alloyed nickel superalloys is found to result in a rough and uneven surface coated with a substantial sludge residue of complex compounds of tungsten, titanium, and molybdenum. The conclusion of experience is that superalloys having substantial amounts of the foregoing elements, as a class, present the most electrolytic etching difficulty.
The most common application of electrolytic etching is the simple smoothing, or electropolishing, of a surface, by the removal of relatively small quantities, i.e., under 25 .mu.m of material from a surface. In other instances, small depressions or grooves are sought. To accomplish this, the workpiece is coated with an insulative material, or resist, in portions not to be removed. Generally, accurate definition is achieved when the depth of the depression is relatively slight. However, when it is sought to create depressions where the depth is appreciable, compared to the surface plane dimensions, it is found that the depressions are widened from the widths defined by the resist. Thus, for example, a groove of 1.0 mm width, as defined by a resist, when etched to a depth of 0.35 mm may be found to increase in width by as much as 60% to a width of 1.6 mm. Furthermore, the side walls of the groove will be tapered outwardly, e.g., the groove will be wider at its top than it is near its bottom. These lateral dimensional effects are characterized as "side etch". Adding to the undesired side etch effect, as grooves are cut deeper the previously mentioned uneveness and roughness are accentuated. Consequently, it is quite difficult within the state of the art to form grooves which have controllable surface finish, uniform depth, and consistent width.
Another problem frequently encountered in electrolytic etching occurs when a workpiece has separated etchable portions with varying surface areas: the larger areas will suffer greater material removal rates than the smaller areas. Therefore, in the absence of special techniques, uneven and uncontrolled depths will result at different locations in the workpiece.
Of course, some of the foregoing problems may be overcome by using various laboratory-type techniques, e.g., separately etching narrow grooves from wide grooves, using different sequences of electrolytes, and so forth, but these are not suited to the requirements of production of a multiplicity of parts. Therefore, there is a need for an electrolytic etching technique adapted to economically create uniform width, depth, and surface finish grooves of substantial depth, especially in nickel superalloys.